1. Field of the Invention
This invention relates to an improvement in the recovery of ethylene oxide (EO) from the gaseous effluent of ethylene oxide reactors, for use as feed to integrated glycol units for the production of polyester grade ethylene glycol (EG). More specifically, this invention relates to improved absorption/stripping systems for use in the EO recovery step of integrated EO/EG plants that will produce a purer ethylene oxide-water feed for the glycol plant, together with substantial savings in both energy consumption and capital investment.
2. Related Information
When ethylene oxide is produced by the silver-catalyzed, vapor-phase partial oxidation of ethylene by molecular oxygen, a hot gaseous reactor effluent is obtained. The ethylene oxide content is quite low, typically in the range of 1-3 mole % and recovery of the ethylene oxide from the effluent gas, as conventionally practiced, involves cooling of the gas in a heat exchanger train and absorption in water, producing a very dilute EO solution.
The function of the water absorption step is to selectively absorb the ethylene oxide from the reaction effluent gas with minimal concurrent absorption of other gaseous materials such as unconverted reactants, reaction diluents, and reaction by-products that are also present. However, since the absorber operates at relatively high pressure, usually only slightly below that of the reactor, and since substantial amounts of absorption water are required, significant amounts of by-product carbon dioxide and undesirable trace impurities such as formaldehyde, acetaldehyde and their acidic byproducts formed during the oxidation step may be concurrently absorbed, along with the ethylene oxide.
In the EO stripper, all of the absorbed ethylene oxide is stripped out using steam, along with all of the carbon dioxide and acetaldehyde, and substantial amounts of the formaldehyde, aldehydic acids, and impurities which may have been absorbed. The overhead vapor from the stripper is then cooled to condense out most of the water, a portion of the ethylene oxide, and most of the formaldehyde, acids and heavy impurities, which are returned to the stripper in some plants. The uncondensed stripper vapor contains the product ethylene oxide which is typically reabsorbed in recycle water from the glycol and EO purification units to produce a more concentrated aqueous solution which is pumped and used as feed to an EO purification system or converted to glycol in a glycol unit.
Even though the stripper typically operates at near-atmospheric pressure, the temperatures within the stripper are high enough to thermally hydrate a small, but a significant portion of the ethylene oxide to ethylene glycol. The glycol produced in the stripper is continuously removed as a dilute glycol-water purge, which is contaminated by formaldehyde, aldehydic acids and other impurities in some plants. The presence of aldehydes and aldehydic acids and impurities in any of the glycol unit feeds are highly undesirable since they result in byproducts in the glycol reactor, which lower the UV quality of the product glycols. It is difficult to upgrade the glycol content in the contaminated purge to polyester (fibre) grade quality, which is preferred, and the prior art includes patented process innovations for reducing the impurity levels in fibre grade glycol plant feeds.
U.S. Pat. No. 3,904,656, uses ion exchange and carbon bed treatment of the stripper bottoms bleed streams to remove acid salts and UV-absorbing impurities prior to adding it to the main glycol evaporation section.
U.S. Pat. No. 3,964,980, (commercially offered by Scientific Design Company Inc.) refluxes the condensed portion of the stripper overhead vapor that contains formaldehyde and aldehydic acids, back to the stripper to produce a higher purity reabsorber bottoms EO solution, which is the feed to the glycol reactor. The stripper bottoms bleed stream is treated as described in U.S. Pat. No. 3,904,656 before being added to the main glycol plant evaporation section.
In U.S. Pat. No. 4,822,926, (commercially offered by Shell Company) the EO reactor effluent from the heat exchanger is first sent to the quench section of an EO absorber. Here it is scrubbed with a recirculated, cooled aqueous alkaline stream to absorb and neutralize acidic compounds such as acetic and formic acids. Part of the trace amount of by-product formaldehyde is also absorbed. A quench bleed is then taken to remove the by-product water, which is partially condensed. The gases from the quench section pass into the main portion of the EO absorber where they are further scrubbed with cold water to recover the EO that is then purified or converted to fibre grade glycol. The quench bleed, which contains typically 0.5-3.0 wt. % of EO and comparable concentrations of glycol and sodium salts can be sent to a quench bleed stripper where EO is stripped out and recovered. The EO-free quench stripper bottoms can then be disposed of as a waste stream or processed separately for technical grade glycol recovery. Alternatively, the quench bleed containing EO can be passed through a pipe reactor to convert the EO to glycol and then processed to recover technical grade glycol and discard the salts, formaldehyde and water.
U.S. Pat. No. 6,417,411 teaches an EO/EG process which incorporates an EO absorber/stripper flow scheme similar to that described in U.S. Pat. No. 3,964,980, in which the stripper bottoms bleed stream is fed to the “by-produced glycol concentration column” where much of the water in the feed is stripped out, producing a concentrated by-product glycol stream that is then processed separately from the main glycol product to recover the glycol content.
The process described in U.S. Pat. Nos. 3,904,656, 3,964,980 and 6,417,411, effectively prevent salts, acids and most of the heavy aldehydic impurities from contaminating the polyester glycol product of integrated EO/EG plants reaction flowschemes, but do not adequately reduce the formaldehyde contamination of the combined glycol plant feed streams. As a result, the formaldehyde concentration in the recycled glycol reaction water builds up and produces undesirably high amounts of heavy aldehydic impurities by reacting with ethylene oxide or ethylene glycol in the main glycol reactor. Many of these heavy aldehydic impurities end up in the ethylene glycol distillate and negatively affect the UV quality of the fibre grade glycol product.
The art therefore has long needed a simple, low-cost and efficient system for EO absorption flowschemes that adequately reduces the formaldehyde content of the feed streams to polyester grade glycol units, eliminates the costly treating of bleed stream feeds and/or product polyester grade glycol, and obviates the need for co-producing technical grade ethylene glycol. To limit the production of less desirable diethylene glycol, all non-catalytic, adiabatic glycol reactors, which are the most usual, operate with very high water/EO molar ratios, FIG. 1, prepared from research data published in the literature, shows that at a 22:1 water/EO molar ratio (10 wt. % EO), the theoretical MEG yield would be about 91.5 mol. %, equivalent to about 92.7 wt % MEG in the product glycols. However, the literature reports that at high water/EO ratios in commercial reactors, the selectivities to monoethylene glycol (MEG) and the MEG yields are “somewhat lower” than those predicted from FIG. 1.
As can be seen from FIG. 1, the beneficial effects on MEG yields of increasing the water recycle start to decrease rapidly above water/EO ratios of 15. Accordingly, in optimizing the design of the glycol plant, the improved MEG yields resulting from increasing the water recycle are balanced against increased capital and utility costs. As a result, commercial non-catalytic plants usually operate with water/EO ratios in the range 18-25:1 (equivalent to EO concentrations of 12-8.5 wt %). Thus, the art has also needed a low-cost and energy efficient way of being able to increase the water/EO molar ratios in the glycol reactor to 30-35:1 to increase the yield of the desired product, monoethylene glycol.
An advantage of this invention is increased production of polyester grade ethylene glycol by reducing the amount of glycol bleeds or off-specification products that are degraded to lower value technical grade products produced when using the prior art EO recovery procedures.